The present invention relates to the field of the sizing of paper and in particular to sizing agents based on an aqueous cationic dispersion of hydrophobic polymers. It also relates to the process for producing such dispersions.
A good sizing agent is desired for every kind of paper, in particular printing and writing paper, paper for labels, wastepaper and wrapping paper.
Two kinds of sizing agents generally exist: surface or internal sizing agents. The term xe2x80x9csurface sizing agentxe2x80x9d is understood to mean an agent applied at the surface of papers, generally in conjunction with starch, using a size press; on the other hand, the term xe2x80x9cinternal sizing agentxe2x80x9d is understood to mean an agent added with the fibres in the wet part of the manufacture of the paper.
A good surface sizing agent allows paper manufacturers to obtain a paper with good resistance to water while reducing and even sometimes eliminating internal sizing, which requires a larger amount of product than surface sizing. It also makes it possible to obtain a paper with good surface properties, which are particularly important in obtaining good printability properties on papers.
It is now known to a person skilled in the art to use, as surface sizing agent, emulsified or dispersed polymers, in combination with starch, in preference to dissolved polymers. They make it possible to obtain sizing agents having a high solids content with a low viscosity. They cause less of a foaming problem than dissolved polymers of the styrene-acrylic acid or styrene-maleic anhydride type, for example. These dispersions can be anionic or cationic, the latter being less common and more difficult to synthesize.
As far as internal sizing agents are concerned, a person skilled in the art is looking for a product which can be used without a retention agent and without a maturing stage. The commonest internal sizing agents are alkyl ketene dimer (AKD), alkyl succinic anhydrides (ASA) and colophonies. They are not suitable under all manufacturing conditions and with regard to all types of pulps and sometimes require the use of a cationic promoter.
Surface sizing agents are known to a person skilled in the art and exist in the form of anionic dispersions. They are copiously described in the literature. They are synthesized by emulsion polymerization using an anionic copolymer of low molecular mass as emulsifier. It is known to a person skilled in the art that the absence of surfactant during the synthesis and a very fine particle size are preferable in obtaining satisfactory effectiveness during the surface treatment. U.S. Pat. No. 08,434,600 discloses, for example, the emulsion synthesis from styrene-acrylic acid copolymers used without cosurfactant to give emulsions with a particle size of 65 nm, while Patent FR 9606737 claims the use of dispersions with a size of approximately 55 nm synthesized using styrene-maleic anhydride (SMA) copolymers.
On the other hand, these anionic emulsions have certain disadvantages. In particular, they show little attachment to cellulose fibres which are slightly anionic, which causes recycling problems in closed circuits. A high dosage is necessary in order to obtain good sizing properties, because a portion of the dispersion does not remain at the surface and spreads throughout the thickness of the paper sheet. Furthermore, such anionic emulsions are not compatible with some starches, such as, for example, cationic starches.
Cationic dispersions have therefore been envisaged as sizing agents. Several methods of synthesis are possible. That which proves to be the most relevant and which the present invention relates to is emulsion polymerization using a cationic copolymer of low mass. Patent DE 2,454,397 claims the use of oligomers based mainly on styrene and on dimethylaminoethyl acrylate which are synthesized in an organic solvent. Patent DE 3,401,573 claims the use of styrene, acrylonitrile and dimethylaminoethyl acrylate or methacrylate terpolymers. These two processes have the disadvantage of using an organic solvent which has to be removed before or after the emulsion polymerization. They are therefore neither economical nor ecological. Furthermore, there may be problems of stability of the emulsion at high temperatures typical of sizing baths or at a high shear gradient. Another patent, Patent DE 4,200,715, claims the use of a hydrophobic modified cationic polyamidoamine. This process results in emulsions with a very low solids content and with particle sizes of about 160 nm. These emulsions are therefore neither economical nor effective in the present application.
According to the prior art, it therefore proves to be impossible to use, as sizing agent, cationic emulsions which have a high solids content, which have a fine particle size and which are synthesized without organic solvent or surfactant.
U.S. Pat. No. 3,444,151 discloses, however, the synthesis of imide derivatives (II) of copolymers based on maleic anhydride (I) and claims their use in emulsion polymerization for the purpose of latex synthesis. However, this patent claims the use of compound (II) in a proportion of 2 to 20% by weight with respect to the monomers, which is in fact insufficient for the purpose of obtaining correctly stabilized latices in the absence of conventional surfactant (the examples taken up in the patent are, moreover, all based on the joint use of conventional surfactant).
The present invention provides a simple process for the synthesis of cationic emulsions and for the application of these emulsions to paper, which give much better results than the equivalent anionic emulsions.
The present invention is distinguished from the prior art in that it uses cationic emulsions of a novel type which are intended to be used as sizing agent, which emulsions are obtained in the presence of imide derivatives (II) of copolymers based on maleic anhydride which are used in sufficient proportions to avoid recourse to conventional surfactants. The emulsions thus synthesized have high hydrophobicity, a fine particle size, a high solids content, weak foaming and good mechanical stability, good stability towards freezing/thawing and good stability at high temperatures. They can be used equally well as surface sizing agent and as internal sizing agent. In the surface application, they are compatible with a large number of starches. In the internal application, they make possible immediate sizing without the use of a retention agent.
The invention therefore relates to the application, as sizing agents for papers, of cationic dispersions resulting from the emulsion polymerization of a mixture of mainly hydrophobic monomers in the presence of an aqueous solution of imide derivatives of copolymers based on maleic anhydride of low molecular mass and in the absence of conventional surfactant. Emulsion synthesis using these copolymers results in strongly hydrophobic cationic dispersions, with a high solids content and with a low particle size, which confer both very good hydrophobicity on the papers treated and good ink-jet printability.